1. Field of the Invention
The present invention relates to an idle speed control apparatus for an internal combustion engine, and more particularly, to an idle speed control apparatus adapted for use with an internal combustion engine provided with a device for recirculating the exhaust gas to the intake or suction system of the engine during idle engine operation, or for use with a cylinder-injection engine adapted to inject the fuel directly into engine cylinders.
2. Description of the Related Art
In order to reduce harmful gas components discharged from fuel-injection spark-ignition internal combustion engines mounted in automobiles and the like, or improve the fuel-efficiency of the engines, various new-type engines (hereinafter referred to as cylinder-injection engines) designed to inject the fuel directly into the combustion chambers thereof have recently been proposed to replace conventional manifold injection engines where the fuel is injected into the intake manifold thereof.
A cylinder-injection engine is proposed in Jpn. Pat. Appln. KOKAI publication No. 5-79370, for example, which is so arranged as to inject the fuel at a timing suitable to the engine load and which has the combustion chambers formed into a shape matching the manner of fuel injection. More specifically, according to the proposed cylinder-injection engine, the fuel injection mode is switched depending on the engine load between a first term injection mode where the fuel is injected in the suction stroke and a second term injection mode where the fuel is injected in the compression stroke. During low-load operation, the fuel injection into a cavity formed in the piston is carried out at the final stage of the compression stroke in such a manner that an air-fuel mixture with an air-fuel ratio near the stoichiometric air-fuel ratio is formed locally around the spark plug or in the cavity, whereby a generally lean mixture can be lit up in the cylinder. This permits a reduction in the emission of CO and HC and greatly improved fuel-efficiency for idle or normal operation of the engine. During mediumload operation, the fuel is injected at the beginning of the suction stroke into the cavity so that an air-fuel mixture is collected in the cavity, thereby permitting stable combustion and improved fuel-efficiency. During high-load operation, the fuel is injected outside the cavity in the suction stroke, so as to form an air-fuel mixture with a uniform air-fuel ratio in the combustion chamber so that the fuel can be burned in a quantity equal to that for manifold-injection engines, thereby producing an engine output required at the time of starting and accelerating the engine.
The proposed cylinder-injection engine permits the overall air-fuel ratio to be set at a considerably large value (e.g., 25 to 40) in the second term injection mode. Further, by supplying a large amount of fresh air to the engine through a bypass passage provided bypassing the throttle valve or by effecting EGR for recirculating a large amount of exhaust gas to the engine, lean combustion can be carried out during low-load operation such as idle operation, to thereby reduce the emission of CO and HC and improve the fuel-efficiency for idle or normal operation of the engine.
According to researches and developments by the present inventors, it has been proved that, for engine output adjustment, the overall air-fuel ratio can be set at an appropriate value by adjusting the fuel injection quantity within a range where no misfires take place, while maintaining the fresh intake air amount and the EGR amount constant, or by adjusting the intake air amount and the EGR amount, while maintaining the fuel injection quantity constant.
This indicates that idle speed control can be achieved by adjusting the fuel injection quantity in the second term injection mode, without making an intake air amount adjustment, thereby attaining improved response in idle speed control, in addition to improved emission and fuel-efficiency characteristics.
However, it has been proved by the present inventors that the idle speed control including a fuel injection quantity adjustment effected in the second term injection mode entails a drawback that the idle speed becomes unstable or smoke is caused when the air-fuel ratio reaches the limit above which a rich accidental fire (a misfire caused by excessively fuel-rich mixture) takes place ff the fuel injection quantity is simply increased with the increase in engine load.
The method of increasing and decreasing the fuel injection quantity so as to control the idle speed can also be realized in the manifold-injection engines. There is a possibility of realizing such idle speed control in a lean-burn engine which is designed to monitor the actual air-fuel ratio with use of a so-called linear air-fuel sensor, or monitor a variation in engine rotation to thereby permit the air-fuel ratio to be set at a value (e.g., 22 to 24) on the fuel-lean side with respect to the stoichiometric air-fuel ratio.
The idle speed control based on fuel injection quantity adjustment for lean-burn engines or cylinder-injection engines, free from fuel transport delay in the intake manifold, is superior in control response as compared with idle speed control based on bypass air amount adjustment, but poses a problem such that it cannot enjoy a wide control range to cope with load fluctuation during idle operation.